Irrigation and Salt

Wind eroded and salt encrusted hoodoos, Atacama Desert, Chile

©EOP

For the past several days I have been wracking my memory and rifling my notes to find the first time I encountered the "3 s" of irrigation - silt, salt and [political] stability. The idea is so straightforward that perhaps the term has been around forever, or at least as long as supplemental water has been used to grow crops. Salt and silt threaten the utility of irrigation works and the soils to which water is applied, while political stability is required if irrigation works are to be maintained over time. Irrigation at any significant scale is a cooperative activity requiring many people to participate in the construction and upkeep of the impoundments to store water, the canals to bring it to fields, and the drains to remove excess water and prevent water-logging of soils. Archaeology and history provide numerous examples of cases where irrigation systems fell into disrepair because soils became too saline for use, because impoundments and canals were clogged with silt, or because political unrest made it impossible to maintain the irrigation works. On occasion the destruction of irrigation works has been a deliberate ploy in warfare as an enemy attempts to starve a population by destroying its agriculture.

Silting behind large dams was noted in the posting on immense dams. It is also a problem behind even the smallest hydraulic barriers and unless water is moving rapidly, it can be a problem in canals as well. In a later posting we shall examine that idea in a bit more detail. In later postings and in the lectures we shall also examine the importance of political stability in the maintenance of irrigation systems.

Today I would like to make a few comments about salt as an enemy of irrigation projects. Salinization is a complex problem for all water systems. Many mineral salts, including the most important single one NaCl or sodium chloride (common table salt), dissolve readily in water. At low concentrations those salts can be be inconsequential or even beneficial for human consumption and agricultural use. At some level of concentration, however, they become a problem, and beyond some maximum concentration the water is no longer useful. We may look at salts other than NaCl later, but for today let us limit discussion to it.

Crop plants vary greatly in tolerance to saline soils and salty water. Grains and grasses tend to be more tolerant than other field crops, while most tree crops are intolerant of saline conditions. When land is being considered for irrigation, an early test is of the natural salinity of the soil recognizing that high value fruit, vegetable, fiber and tree crops are unlikely to thrive if the soil is already quite saline. Grain crops in general do not return enough value per hectare to justify investment in irrigation facilities.

Salts accumulate in soils through a variety of processes, and some soils are naturally saline, especially ones derived from rock substrates containing high levels of salt. Elsewhere in arid areas evaporation of surface ponds and lakes leads to pockets of salty soil (often called playas in Mexico and the southwestern US  and salinas in South America). Once overlain by glaciers, with numerous small ponds and lakes left as the glaciers receded the now semi-arid southern part of the Canadian province of Alberta, its agricultural area, has salty soils spread quite widely. Some of those soils derive from salty substrates, while other areas were once covered by waters now evaporated away. As grains and grass fed livestock are the primary agricultural products, the presence of salty soil is not a great hindrance to agriculture in Alberta (uncolored areas on the map are generally unsuitable for agriculture for other reasons, while blue areas are lakes and streams).

Source: Agriculture Canada and Province of Alberta

Commonly soils become salty after irrigation commences. Not infrequently in arid areas the water used for irrigation is itself  saline. Rainwater dissolves salt as it flows across desert landscapes in infrequent storms. Water captured in impoundments becomes increasingly saline as evaporation occurs. The extremely high rate of evaporation in hot desert areas like those of the southwestern US mean that water leaving Lake Powell or Lake Mead is far saltier than water flowing into those reservoirs. Applied to the soil in areas with high rates of evaporation, some salt from that water is added to the soil with irrigation. Much of the salt is leached to lower levels in the soil profile, and some of that, in turn, is washed away. But water retained in the soil can be  returned to the surface in a capillary process as water is drawn upwards to the soil surface by evaporation.

Rain or irrigation, in the absence of leaching, can bring salts to the surface by capillary action

Source: Wikipedia Media Commons

Only if it is possible to flush the salts by using a large quantity of water to redissolve the salts and thus to remove them can the eventual salinization of the soils be prevented. Accumulation of salt in soils has rendered substantial areas once productive agricultural zones into salty desert too saline for crops. In the Middle East and in arid parts of Asia, many square kilometers of once productive agricultural activity have been abandoned because of salinization. 

Source: Australia Department of Agriculture, Fisheries and Forestry, Bureau of Rural Sciences

While irrigation agriculture is fairly new to Australia, the Murray River Basin in the southeastern quadrant has been irrigated for more than a century. Not surprisingly, that arid area is reporting salinity problems as the map illustrates.